Rheology is the study of deformation and flow of matter (1). In food applications, deformation is usually involved with assessing texture, while flow is generally associated with the property of viscosity (2). Generally texture is more loosely defined as "the response of the tactile senses to physical stimuli that result from contact between some part of the body and the food ... a group of physical properties that derive from the structure of the food . . . properties expressed in terms of mass, distance and time only." In cheese grading, texture may refer to appearance while body may be the term used to denote rheological properties. Here, texture is used as a rheological term. Viscosity can be more rigorously defined as "the internal friction of a fluid or its tendency to resist flow." Often texture is used when describing the rheological properties of solid or semisolid foods such as cheese, fruit, bread, or cold ice cream, while viscosity is generally applied to foods that flow, such as pourable salad dressings, vegetable oils, syrups, or melted ice cream. Texture can also be used with a broader meaning, synonymously with rheology, including viscous properties of the food; this meaning of texture is especially common in describing semisolid foods such as ketchup, mayonnaise, or purées. Of the many sensory food attributes of concern to a food scientist, texture is the least well described (3). It has been observed that the consumer may also have difficulty describing texture but this difficulty does not lessen the importance of this attribute:

When first asked about food texture, the consumer appears to exhibit very little spontaneous awareness. Flavor overshadows texture at the conscious level. People simply take the texture of a food for granted. . . . An average consumer may have difficulty in visualizing the concept of texture per se. . . . If the texture of a food is the way people have learned to expect it to be, and if it is psychologically and physiologically acceptable, then it will scarcely be noticed. If, however, the texture is not as it is expected to be ... it becomes a focal point for criticism and rejection of the food. Care must be taken not to underestimate the importance of texture just because it is taken for granted when all is as it should be (4).

The science of rheology made significant advances since 1940 in the determination and explanation of the viscous and viscoelastic properties of polymers; this part of the field had its origins in the time of Newton and now has a solid theoretical and experimental basis (2,5,6). Analysis of the texture of foods is a more recent application of rheology. Modern food texture analysis could be said to have begun with the development of texture profile analysis (TPA, a scheme for measuring and classifying textural properties) (3,4,7-12). A texturometer (9), which evolved from a denture tenderometer built at MIT (13), has largely been replaced by tensile-compression testing machines made by Instron Corp. (14) and other similar instruments.

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